: Christopher Tolkien says in THE PEOPLES OF MIDDLE-EARTH that the notion of Gil-galad being Fingon's son was never more than "an ephemeral idea". It is simply impossible for him to be Fingon's son. He was not always Orodreth's son, but he was almost always a member of the House of Finarfin, originating as Finrod Felagund's son.

Of course the other thing to note is that JRRT never published a
a final geneology for the Noldor. His last conceptions before he died were that Gil-galad should be son of Orodreth but had he lived he may very well have changed his mind yet again (as his was wont to do) for nothing definitve was actually published that he would have felt bound to remain with. He could have made Gil-gald another son if Fingolfin, changed him to a son of Finarfin, left him as Orodreth's son or returned him to his place as Fingon's son. No one can say with absolute authority what he would have ended up doing, we can only make 'educated' guesses and any ones opinion is as good as the next. So of the several conceptions that the professor had at various times you are free to choose what best fits your own picture of the tales.

: On the continuing question of kingship and inheritance (which may never be satisfactorily settled),

Probobly not but it has led to an interesting debat which has probobly got us a looking at the material in differt ways and yet finding new things we had not noticed before (such as the successor statement and all the interesting words to be found in the 'Etymologies'). :):)

:I found the following entry in "The Etymologies". It is therefore dated information but seems to me to be worth mentioning, as probably having been retained by Tolkien into the 1950s and 1960s. This is a subentry under "TA-, TA3-":

:

:   *taro  king: only used of the legitimate kings of the
:   whole tribes, as Ingwe of the Lindar, Finwe of the Noldor
:   (and later Fingolfin and Fingon of all exiled Gnomes).  The
:   word used of a lord or king of a specified region was
:   aran (âr), Q haran [see 3AR].  Thus
:   Fingolfin taur egledhur 'King of the Exiles' [see LED],
:   but Fingolfin aran Chithlum 'King of Hithlum'.  Q
:   tár (pl. tári).  N taur,
:   Ilk. tôr, only used of Thingol: Tor Thingol
:   = King Thingol.
: 

: If I understand the language relationships correctly, "Q" represents "Qenya", the predecessor language for Quenya, spoken only in Aman. "N" represents Noldorin, the predecessor language for Sindarin, spoken by the Exiles, and "Ilk" represents Ilkorin, spoken by the Elves of Beleriand (and perhaps other lands, but mostly Thingol's people and the Danas, who became the Green-elves of Ossiriand). Ilkorin would have become the Doriathian dialect of Sindarin.

: As I've pointed out elsewhere, there is not a 1-to-1 correlation between these languages and the later languages, but they are very similar and Tolkien did in fact retain the name "Noldorin" for what became "Sindarin" even up until he was working on the Appendices for THE LORD OF THE RINGS, and only then made the name changes.

: An entry "NDEW-" for "follow, come behind" does exist in "The Etymologies", and one of its derivations was "Dior, successor (i.e., of Thingol)", but I don't know if these entries are contemporary, or if the omission of the title "Tor Dior" was due to oversight or in some way deliberate (and if so, was it deliberate only in the sense that Tolkien felt one example was sufficient, or in the sense that Dior did not succeed Thingol as Tor but only as Aran?).

JRRT doesn't mention Dior as either 'ARAN' or 'TOR' in the etymologies. He just says that Dior is the Successor of Thingol.
However he does note Thingol as both 'ARAN' and 'TOR'. It is interesting to note however that Dior if referred to as the 'King IN Doriath' (emphasis mine).

: This entry, however, does indicate that Dior's name was more a literary convention than anything else, although it would not be impertinent to infer some sort of prophetic association may have been intended.

I agree about the prophetic portion. As JRRT said it was not uncommon for the Eldar to give their children prophetic names.
Also in reference to what you state below about 'Eluchil' it should be noted that as a name it does not appear in the work associated with the Etymologies (ie 'The Lost Road'). Dior therin is simply referred to as the heir of Thingol.

: An entry for "khil-" is carried over into the Elvish dictionary in THE SILMARILLION, and it is said to mean "follow". Although "Eluchil" is translated in its own index entry as "Heir of Elu (Thingol)" a more literal translation would be "successor of Elu". Dior's early name was "Dior Aranel" (Dior Royal-elf) and he is only said to have later been called "Dior Eluchil". Use of "heir" interchangeably with "successor" here seems to indicate an understanding that he who followed in the other's footsteps was taking up or receiving the heritage left of the predecessor.